This machine follows a very simple logic: single higher denomination equals the equivalent number of the next highest denomination of coin. So, a twenty pound note and a ten pound note are converted into two pound coins, and two pound coins are converted into fifty pence pieces. If you want one pound coins, use the machine next to this one which follows the same logic.
As user interfaces go, it takes a bit of thinking about at first. We expect things like this to be more complicated. But once you understand the rules it makes sense. It transfers the decision-making to the user, you’ve got to get the input right to get the output you want, the machine doesn’t figure that out for you, but the simplicity of functioning makes figuring that out quite easy.
Software is someone else’s decisions. The simple mechanical logic of this machine relies on the fewest of those decisions for the user to achieve want they want to.